This invention relates to a semi-soft handoff method and system that uses multiple common frequencies, and more particularly, to a system and method of executing inter-cell inter-frequency assignments with semi-soft handoffs in mobile communication systems employing Code Division Multiple Access (CDMA).
A Personal Communications Services (PCS) and/or CDMA cellular system typically include a Base station Transceiver Subsystem (BTS), a Base Station Controller (BSC), a Base Station Manager System that manages the operation of many BSC""s, a Switching system and a Location Registration System. Each base station transceiver subsystem usually manages one cell.
When a wireless telephone (mobile station or unit) moves to various areas, communication transmission difficulties should be minimized, and when the mobile station is idle, the mobile station must regularly re-register according to various parameters. When a call is in progress, the mobile station, the base station and the switching station manage the communication between the mobile station and the base station to maintain an acceptable radio link efficiency.
In a CDMA and wide-band CDMA system, one mobile station can simultaneously receive signals from more than two base stations. Therefore, the CDMA and the wide-band CDMA system can process a handoff from one base station to another base station or from one antenna area to another area within one base station.
As for the CDMA cellular and the PCS system, to guarantee the call""s continuance, various forms of handoffs are offered. These various forms of handoffs can have efficiency differences with respect to their reliability regarding a call""s continuance and the system load which follows from the method of handoff and from the contents of the call.
As is known to those skilled in the art, the handoff method is classified into mainly soft handoffs and hard handoffs. Soft handoffs are classified into inter-cell soft handoffs, inter-BSC soft handoffs and inter-sector soft handoffs. In particular, the inter-sector handoff is often called softer handoff. Hard handoffs are classified into inter-MSC (mobile station switching center) hard handoffs, inter-frequency assignment hard handoffs and inter-frame offset hard handoffs.
In addition, there is an inter-cell inter-frequency handoff in the CDMA network which is employed when the serviceable frequency channel""s number does not accord with adjacent cells. With respect to this type of system, the inter-cell inter-frequency handoff is categorized into an inter-cell inter-frequency hard handoff that uses a pilot generator and a semi-soft handoff that uses a common frequency.
Although the inter-cell inter-frequency hard handoff, which uses the pilot generator, requires inter-cell handoff, it is also a method of directly transferring to another frequency within a destination cell in the case where the frequency executing the present call cannot receive the service within the destination cell to be moved to. For additional hardware, like the pilot generator, etc. that must be setup in the system, space is needed within the system and must be provided with the necessary power and signal/power etc. interface(s). Furthermore, for one pilot generator, there is a need for a channel card, a sector interface card, a transceiver module and a high power amplifier etc. If the pilot generator were miniaturized for use in external equipment, it would become much more difficult to provide an interface between the external pilot generator and the inter-cell system""s interface. In this case, to combine the pilot generator""s final output to the system""s output, additional modules like RF power couplers, etc. are needed. Because these additional modules add expense to the entire system, a system redesign considering such changes is required. To compensate for the additional power loss, the system output level must also be increased. Limitations in the performance and in the development of a miniaturized mobile system are brought about by these problems.
The Semi-Soft Handoff, which uses a common frequency, processes the handoff by software means. It uses a primary channel with the common frequency when the adjacent inter-cell frequency assignment is different. However, unlike the inter-cell inter-frequency hard handoff which uses the pilot generator to verify the presence of an adjacent base station, the semi-soft handoff method determines the handoff point based only on the weakening of the present cell""s pilot signal power. However, because the handoff method does not require additional hardware, it is beneficial with regard to system price, use of space and facility maintenance repairs.
With a semi-soft handoff method, because the inter-frequency hard handoff within the cell occurs toward the common frequency, the decisive parameter required for the handoff is the amount of the common frequency""s load. If the amount of the common frequency""s load is excessive, additional requests for inter-frequency hard handoffs within the cell cannot be processed. Also, the assignment of a new call to the common frequency and a soft handoff from the adjacent base station cannot be processed. Therefore, whether the common frequency will be overloaded can be known by comparing the hard handoff call""s load occurring within the cell and the common frequency processing function amount for the entire call processing system.
Therefore, a need exists for an improved handoff method which employs multiple common frequencies to address problems occurring due to common frequency overload.
The present invention was devised to solve problems in the semi-soft handoff method. The object of the invention is to offer a semi-soft handoff method that uses multiple common frequencies.
To achieve the above objective, according to the invention, there is provided a semi-soft handoff method, using multiple common frequencies. A base station makes use of a multiple frequency channel. When a mobile station moves inter-cell, and the service on the frequency that processes a present sector within a destination cell to be moved to is not offered, to guarantee the call""s continuance, the base station examines all adjacent cells"" frequency assignment conditions and detects frequencies that all cells have commonly and designates the detected frequencies as all cells"" common frequency. The traffic is managed within the common frequency. When a handoff request message is received to a base station, the base station detects a common frequency with a lowest load among the common frequencies. The base station executes an intra-cell inter-frequency hard handoff to the common frequency with the lowest load, and an inter-cell soft handoff to the destination cell by using the common frequency. An inter-cell inter-frequency hard handoff is executed to a corresponding common frequency. A soft handoff is executed to the destination cell by using the common frequency determined by the examination of minimum load.
In the present invention, it is preferable that each base station individually executes inter-cell inter-frequency handoffs using multiple common frequencies, that the inter-cell inter-frequency handoff is determined based on the power of the pilot signal receiving the present service when the hard handoff is executed. It is also preferable that the number of the common frequencies is set at the lowest among frequencies assigned to each cell. When a common frequency is overloaded, calls of the overloaded common frequency move to another frequency by forcing a hard handoff. It is further preferable that the base station designates a paging channel of an idle mobile station within the common frequency as a noncommon frequency so as to avoid new traffic from being assigned to the common frequency. When the idle mobile station tries to form a traffic channel, the base station assigns traffic for the mobile station to a traffic channel in the noncommon frequency. The base station manages the load of traffic for each common frequency for traffic uniformity. Accordingly, the base station assigns traffic from new calls to a common frequency with the lowest traffic load. A function is provided to set up a call by assigning the traffic channel to a different frequency, not to the corresponding mobile station""s frequency range when setting a new call. The sector is set to another frequency by designating a CDMA frequency""s description to an applicable frequency, which is among the descriptions of the paging channel CDMA assigned messages.
In one preferred mode, in accordance with the present invention, the semi-soft handoff method, in the instance a base station makes use of multiple frequency channel, a mobile station moves inter-cell and service on a frequency that processes the present call within the destination cell to be moved to is not offered, to guarantee the call""s continuance, the base station examines all adjacent cells"" frequency assignment conditions and detects frequencies that all cells have commonly and designates the detected frequencies as all cells"" common frequency. Furthermore, the base station manages the traffic within the multiple common frequencies. After receiving a handoff request message, the base station detects a common frequency with a minimum load among the common frequencies. The base station executes an intra-cell inter-frequency hard handoff to the common frequency with the lowest load and an inter-cell soft handoff to the destination cell by using the common frequency. Next, the base station executes an inter-cell inter-frequency hard handoff to the corresponding common frequency and soft handoff to the destination cell by using the common frequency.